The present invention relates to a wavelength modulator, and more particularly, it relates to a wavelength modulator which modulates a laser beam to a second harmonic component at high efficiency.
A fiber wavelength modulator device as illustrated in FIG. 5 B has gained popularity in practical use. It comprises a core [10] of an organic non-linear optical crystal having a secondary non-linear optical effect and a cladding [11] of a glass showing no secondary non-linear optical effect. In such a fiber wavelength modulator device, Cerenkov radiation is generated so that a secondary harmonic component is radiated from the core [10] to the cladding [11] making an angle .THETA., provided that this .THETA. satisfies the following equation: EQU cos .THETA.=n.sub.eff (.omega.)/N(2.omega.) (1)
where, n.sub.eff (.omega.) represents the effective refractive index of core [10] with respect to the fundamental wave; and N (2.omega.) represents the refractive index of the cladding [11] with respect to the secondary harmonic component. In such a case, the emitted secondary harmonic component gives a ring pattern [14] as shown in FIG. 5. In FIG. 5, the fundamental wave is shown by arrow [13].
Conventionally, a glass cladding [11] was employed having a relative difference between the core [10] and the cladding [11] in the refraction index with respect to the fundamental wave of 0.010 or less. Here, the relative difference between the core and the cladding in refractive index above is defined by the following equation EQU (n(.omega.)-N(.omega.))/ n(.omega.) (2)
wherein, n(.omega.) represents the refractive index of the core [10] with respect to the fundamental wave; and N(.omega.) represents the refractive index of the cladding [11] with respect to the fundamental wave. Further, the diameter, a, of the core [10] is specified so as to satisfy the following equation EQU a&lt;2.405.lambda./.pi.n(.omega.).sup.2 -N(.omega.).sup.2 ( 3)
wherein, .lambda. represents the wavelength of the fundamental wave.
Thus, by transmitting the fundamental wave in its single mode through the core [10] set up as above, it is possible to improve the efficiency of modulating the fundamental wave to the second harmonic component (hereinafter referred to as `modulation efficiency`), and also to obtain a sharp single ring [14] as the output pattern of the second harmonic component.
In a setup comprising a single crystal of 4-(N,N-dimethylamino)-3-acetamidonitrobenzene (DAN) as the core [10], for example, the DAN single crystal is arranged in the cladding [11] in such a manner that the dielectric principal axes X, Y, and Z may be oriented along specific directions as illustrated in FIG. 6. When an incident beam polarized along the c-direction enters the core [10] above, a secondary harmonic component polarized along the Y-axis direction is emitted. In this case, the refractive index of the core [10] along the c-direction is 1.738 with respect to a light of 1064 nm in wavelength.
If an SF4 glass having a refractive index of 1.726 with respect to a light 1064 nm in wavelength is used in combination as the cladding with the crystal mentioned above, the resulting fiber wavelength modulator device B is one having a relative difference between the core and the cladding in refraction index of 0.0069. Thus, when a laser beam 1064 nm in wavelength polarized along the c-direction emitted from an Nd:YAG laser at the output power of 100 mW enters the core [10] of the aforesaid wavelength modulator device B, a second harmonic component 1.4 .mu.W in output is obtained.
The intensity of the modulated beam which is emitted from the fiber modulator device B is linearly related to the second power of the power density, P.omega./a (where, P.omega. represents the power of the fundamental wave, and a represents the diameter of core [10]), of the fundamental wave in the core [10]. Accordingly, there is proposed to reduce the core diameter, a, to a range of from 1 to 3 .mu.m and thereby improve the modulation efficiency by increasing the power density, P.omega./a.
When the diameter, a, of the core [10] is reduced, however, the power, P.omega., of the fundamental wave inside the core [10] also decreases, and this reversely works in improving the modulation efficiency.
An object of the present invention is therefore, to provide a wavelength modulator device which emits a second harmonic component from a fundamental wave at high efficiency.